洞口防护的盖板须能保持四周搁置均衡，并有（）的措施。
A、警示标示
B、固定器位置
C、将洞口全部盖没
参考答案：B
______delighted me most was________my son was admitted to Beijing University.
A．That, what
B．That , that
C．What, that
D．What, what
简述弹性体唇形密封的密封界面特征？
